Pulsed laser device employing electrodes with projections

ABSTRACT

Electrodes for and a stabilized pulsed laser utilizing electrodes for providing a gas discharge created by a pulsed electric field. At least one of the electrodes is made uniformly rough on a scale small compared to both the interelectrode spacing and electrode width as by using freestanding wires to generally define a wire brush arrangement.

United States Patent Leonard et al.

[451 Jan. 18,1972

[54] PULSED LASER DEVICE EMPLOYING ELECTRODES WITH PROJECTIONS [72]Inventors: Donald A. Leonard, Stoneham; Henry W.

Smith, Essex, both of Mass.

[73] Assignee: Avco Corporation, Cincinnati, Ohio [22] Filed: July 10,1968 [21] App1.No.: 743,867

[52] US. Cl ..33l/94.5, 313/351 [51] Int. Cl ..H0ls 3/00 [58]FieldofSeareh. ..33l/94.5;3l3/35l,2l7

[56] References Cited UNITED STATES PATENTS 2,929,922 3/1960 Schawlow etal. 331/94 5 3,149,290 9/1964 Bennett, Jr. et al 331/94 5 3,253,2265/1966 l-lerriott et al. ..331/94.5 3,388,314 6/1968 Gould ..331/94.53,396,301 8/1968 Kobayashi et a1 ..33l/94.5

2,697,800 12/1954 Roberts ..313/351X 2,860,276 11/1958 Grogg, Jr. et al...313/351 3,174,043 3/1965 Dyke et a1. ..313/351 UX 3,402,313 9/1968Gabor et al. ..313/351 X 3,466,485 9/1969 Arthur, Jr. et al. ..313/351 XOTHER PUBLICATIONS Herriott, J.O.S.A., Vol.52, No. 1, H62, pp. 31- 37Chebotayev, Radio Eng. & Elec. Phys., Vol. 10, 2/65, pp. 316- 318 Patel,Phys. Rev. Ltrs., Vol. 13, No.21, 11/64, pp. 617 619 PrimaryExaminerRonald L. Wibert Assistant ExaminerWarren A. SklarAttorneyCharles M. Hogan and Melvin E. Frederick [57] ABSTRACTElectrodes for and a stabilized pulsed laser utilizing electrodes forproviding a gas discharge created by a pulsed electric field. At leastone of the electrodes is made uniformly rough on a scale small comparedto both the interelectrode spacing and electrode width as by usingfreestanding wires to generally define a wire brush arrangement.

8 Claims, 4 Drawing Figures PATENTED JAN 1 8 m2 sum 1 or 2 WMZZZZZWWAATTORNEYS PATENTEUJAM: 81972 DONALD A. LEONARD HENRY W. SMITH ATTORNEYSPULSE!) LASER DEVICE EMPLOYING ELECTRODES WllTi-I PRUJECTIONS Thisinvention relates to pulsed laser devices and more particularly to laserdevices which employ electrodes that are uniformly rough on a scalesmall compared to both the electrode spacing and electrode width.

In the operation of prior art pulsed nitrogen and pulsed neon lasers, aninstability in the discharge current distribution leading to aconcentration of current in one local region occurs with a disturbingdegree of regularity. This concentration of current into an arc spotresults in decreased power output and quite often leads to eventualdestructive failure of the electrodes and/or dielectric sidewalls of thelaser channel.

A crossed-field geometry which has been developed for the pulsednitrogen and pulsed neon lasers is described inpatent application Ser.No. 536,094 filed Mar. 21, 1966, now US. Pat. No. 3,553,603 to whichreference is made. See also the article entitled The S40l-A Pulsed NeonLaser by Donald A. Leonard, published in IEEE Journal of QuantumElectronics, Volume QE-3, Number 3, March 1967, pp. 134-135.

In the aforementioned prior art crossed field type of laser, power flowsfrom a capacitor (see FIG. 2) through low inductance transmission linesto the upper electrode, which typically is a single smooth metal striprunning the length of the active region. A U-shaped channel serves bothas structural support for the device and as the other electrode. Thedischarge takes place between dielectric sidewalls. On the short timescales required by these lasers, the initial current distribution isessentially inductance-controlled and, when operating normally and notsubject to are spot or hot spot failure, extremely uniform dischargescan be produced along the entire length of the channel.

Arc spot or hot spot failure mode in these prior art lasers may be seenas a bright spot or several bright spots which appear at randompositions along the channel. With aluminum channels and aluminumelectrodes, upon examination after operation with hot spots, apronounced pitting and discoloration of the electrodes will'be observed.In the development of the present invention, stainless steel, copper andchrome electrodes were tried but were found to show similar effectsdifiering only in detail-to some extent. However, in general, theseelectrodes were subject to the same are spot or hot spot failure mode.The position of pits on electrodes correlated with the position of theobserved hot spots when the laser was operatmg.

As a result of the above-noted and additional trials to eliminate hotspots, it was found that the onset and behavior of hot spots is afunction of gas pressure in the laser, the repetition rate, the averagepower being dissipated in the laser, the temperature of the channel, andthe material and surface condition of the electrodes and the sidewalls.In general, the appearance of hot spots was found to be favored byhigher repetition rates, higher average power, and higher operatingtemperatures. The process of, hot spot formation also was found to besomewhat irreversible-once a hot spot appeared, even if the laser wasshut off, allowed to cool down and then started again, the hot spottended to reappear in the same spot. This led to the conclusion that thesurface condition of the electrode is important and that pits or roughspots in an otherwise smooth and uniform surface favor the formation ofhot spots. The operating pressure was also found to be a significantfactor in the onset and behavior of hot spots. The aforementionednitrogen laser, for example, normally operates at a pressure of 25 torrof nitrogen gas. If this pressure is either raised or lowered by about afactor of 3, i. e., to below torr or to above 75 torr, then hot spotstend to readily appear and, again, if the laser is shut down andrestarted at the correct pressure, the hot spots still tend to reappear.Accordingly, in the aforementioned prior art lasers, operation at awrong pressure setting can thus be a fail-catastrophic condition.

A typical life test for alaser as disclosed in the aforementionedapplication Ser. No. 536,094, having a 1 meter long aluminum electrodeand aluminum channel, both watercooled, operating at pulses per second,having nominal output of 100 kilowatts peak laser power and a powerdissipation of about 200 watts in the channel, is about 40 hours. Insuch a laser, hot spots generally develop after about 40 hours ofoperation with a coincident fall-off in power. By way of example, in oneparticular test a hot spot that occurred at about the 38th hourdisappeared by the 42nd hour but was replaced at the 52nd hour by a hotspot that got progressively worse from that point on while the testcontinued.

Before development of an improved laser in accordance with the presehtinvention, which improved laser is substantially immune to hot spotfailure, various other attempts were made to I prevent the formation ofhot spots. For example, channels and electrodes were first water-cooledand this obvious measure extended the time to failure by hot spots byperhaps a factor of 2. Also, care was taken with the surface finish ofthe electrodes to obtain as smooth a surface as possible. However, thisdid not seem to influence performance significantly, as pits andimperfections occurred regardless of surface finish. Various materialswere triednotably chromeplating of the electrodes and the use of quartz,dielectric walls. Chrome was chosen because of its hardness and lowvapor pressure. The performance of the chrome-plate-quartz-walledcombination was unexpectedly poor with hot spots developing within a fewhours. Apparently a regenerative process developed whereby a hot spotonce started was able to increase in intensity and thereby reach a veryhigh temperature because of the high temperature capability of chromeand quartz. In channels containing the aforementioned chrome and quartz,the hot spot position correlated with the position of deep pit marks inthe chrome.

In accordance with the present invention, hot spot failure issubstantially eliminated by making at least the upper electrodeuniformly rough on a scale small compared to both the interelectrodespacing and the electrode width so that currents originating fromadjacent rough points will diffuse and merge together and thereforeproduce a uniform current distribution in the body of the dischargechannel. The scale, character and spacing of the rough points should,however, be larger than, and not perturbed or distorted by imperfectionsor temporary arc spots or pitting that may from time to time occur underdischarge conditions. Achievement of this effectively stabilizes thecurrent distribution and insures that small local current concentrationsthat may occur from time to time do not produce significant local pitsor hot spots on the electrodes which in turn cause a currentconcentration at that spot. By producing in accordance with theinvention an electrode 'with essentially as many as hundreds ofuniformly distributed imperfections per inch, a laser incorporating suchan electrode is stable with respect to the formation of new andundesirable imperfections from other uncontrolled causes. A smooth priorart electrode on the other hand is sensitive to imperfections, since animperfection on a smooth surface is a unique point.

The novel features that are considered characteristic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of a specific embodiment when read inconjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of an embodiment of the invention takenpartially in section showing portions of the invention in detail;

FIG. 2 is a schematic view showing the electric circuit employed in thedevice of FIG. 1;

FIG. 3 is a side-elevational view partially in section showing thedevice of FIG. 1 having portions of the structure omitted; and

FIG. 4 is a sectional view taken along lines 4-4 of FIG. 3 showingelements of the invention in detail.

Referring to the drawings, especially FIG. 1, there is shown a laserdevice generally designated by the numeral 10 and having a base 11mounted on a pair of supports 12 and 13. The

base 1 1 is a U-shaped channel member (see FIG. 4) fabricated from anysuitable conductive material, such as aluminum.

The base member 11 having two-leg portions 11a--11b and a bight portion110 interconnecting the legs as shown in FIG. 4, serves both as astructural member as is evident from FIG. 1 and an electrode asschematically depicted in the circuit shown in FIG. 2.

Referring now back to FIG. 1 taken in conjunction with FIGS. 3 and 4,the laser device is further comprised of a pair of sidewalls 14a--14band l6a-16b fabricated of a suitable insulating material such as quartzor Pyrex glass. The outermost sidewall members 140 and 16a have arelatively thin upwardly extending flange. Each sidewall is comprised ofan outer member and an inner member to facilitate and simplifyfabrication and is disposed in the base 11 with the respective steppedsurfaces oppositely facing each other as is best shown in FIG. 4. Anelectrode member 17 (more fully described hereinafter) is supported bythe sidewalls and disposed between the aforementioned upwardly extendingflanges of sidewall members 140 and 16a, resting on the top surface ofmembers 14b and 16b. In addition to being supported in interfittingengagement between the sidewalls, the electrode member 17 serves tomaintain the sidewalls in spaced relation and the flanges of members 14aand 16a prevent arcing between electrode member 17 and base member 11. Aplurality of spacers 18 may be disposed adjacent the bottom surface ofthe base 11 to maintain the lower portions of the sidewalls spaced onefrom another. The space thus provided between the sidewalls defines arelatively thin elongated duct 19 extending from adjacent one end of thelaser device 10 to the other end thereof.

A pair of gas-feeder tubes 21 and 22 are located on the bottom surfaceof the base 11 and provide passages through the lower portion of thebase 11, opening into the cavity 19.

Referring specifically to FIG. 1, there is shown a pair of panels 23 and24, one disposed adjacent each end of the sidewalls and serving to sealthe ends of the cavity 19. The

panels 23 and 24 are transparent to light at the wavelength of radiationproduced in the cavity 19 and thus serve as windows for the laser beamemanating from the cavity. The panels 23 and 24 may be fabricated fromquartz or other well-known material having the desired properties asstated.

It should here be noted that the cavity 19 is generally maintained at apressure other than atmospheric such as, for example, about 25 torr,during the operationof the laser device 10. As is obvious, therefore, itis necessary to pressure-seal the means defining the cavity 19. Thevarious contacting surfaces between the sidewalls and the base 11, theelectrode member 17, and the panels 23 and 24 are therefore sealed by asuitable cement or sealant material to provide at least a relativelygastight enclosure in the cavity 19.

Referring still to FIG. 1, adjacent the translucent panel 24 there isdisposed a mirror 26 having its reflective surface in contact with thepanel 24. The mirror 26 may be a first surface-silvered mirror and maybe cemented to the panel 24 along its edges or held in place by anyother suitable means leaving the reflective surface unobstructed.Alternatively, panel 24 may comprise the mirror.

The electrode member 17 is provided with a plurality of jack receptacles27 which are equally spaced along the length of the member. A covermember 28 fabricated of open metal grill work or other suitableconductive material is provided with a plurality of jacks 29 spaced forengagement with the jack receptacles 27 when the cover is positioned onthe base 11. The cover member 28 may be removably attached to the base11 asby bolts31.

With the cover 28 in place, the end plate 32 is attached adjacent theclosed end of the laser device 10 and a port plate 33 is fastened to theopen end of the laser device.

The port plate 33 has provided therein an elongated opening 34 inalignment with the cavity 19 and substantially equal in area to thecross section of the cavity.

In FIG. 2 there is schematically shown means for applying a pulsedelectric field across the cavity 19.

Referring now to that FIG. 2, taken in connection with FIG. 1, it willbe noted that each of the jacks 29 are connected through the covermember 28 to a plurality of coaxial cables 36. The cables 36 may beconnected through a triggered spark gap 37 or thyratron to a capacitormeans 38, generally a capacitor bank. The energy for the capacitor means38 is provided by a high-voltage supply which is placed in series with acurrent-limiting resistor 39. The coaxial cables 36 are groundedto thecover member 28 and serve to complete the circuit through the covermember and the base 11 to the gas in the cavity 19.

The spark gap 37 is triggered by a pulse circuit which may be acommerciallyavailable device well known in the art. With the deviceshown, the circuit parameters are typically L=0.02 microhenries, C=0.03microfarads, with an initial capacitor voltage of 15 to 25 kilovolts.

Returning now to the electrode member 17, it is provided along itslength with a passage 40 for receiving a coolant such as water. Theelectrode member 17 is also provided along its length with a recess 41in its lower surface in register with cavity 19. Disposed within recess41 are outer spacer members 43 and 44, a center spacer member 45, ascreen member 46 disposed between and separating spacers 43 and 45, anda screen member 47 disposed between and separating spacers I 44 and 45.The outer spacers 43 and 44 extend slightly below the lower surface ofelectrode member 17 to present shoulders to the inner sidewall members14b and 16b and thereby function as spacers to not only maintain thescreen member away from the sidewalls a short distance, but to alsomaintain the aforementioned inner sidewall members in spacedrelationship. Setscrews 48 spaced along the length of electrode member17 are provided to engage one of the outer spacer members and therebysecurely lockthe spacers and screen members in the electrode member 17.If desired, to improve heattransfer from the screen members, they may besoldered into groove 41. Relatively closely spaced and elongatedprojections 46a and 47a, such as for example freestanding wires orwirelike projection; extend past the surface of the electrode member 17and into cavity 19. The mean distance between the extreme ends of theseprojections and surface 50 of base 11 define the electrode-spacing.While the length of the projections intermediate the ends of cavity 19are preferably the same length, those immediately adjacent the ends ofcavity 19 should gradually decrease in length to prevent high electricfield concentrations at these locations. Satisfactory projectionsproviding continuous operation in excess of 200 hours have beenfabricated from No. 32 gauge, 30x30 stainless steel wire screen with sixstrands of horizontal wire removed to provide projections approximatelyone-eighth inch in length.

While for reasons of convenience, economy and heat transfer, the screenmembers may be fabricated from stainless steel screening and the like,the invention is not so limited. The operational surface of electrode 17(and surface 50 if desired) exposed to cavity 19 however provided, needonly'be uniformly rough on a scale, small compared to both the effectiveinterelectrode spacing and electrode width, so that currentsoriginatingfrom adjacent rough points or projections will diffuse and mergetogether and thereby produce a uniform current distribution in the bodyof the discharge channel or cavity 19, the scale and character of theroughness or projections being such that they are not substantiallyperturbed ordistorted by imperfections, heating and/or temporary arcspots or pitting that may from time to time occur under dischargeconditions. The preceding provides stabilization of the currentdistribution in the discharge channel and insures that small localcurrent concentrations that may occur from time to time do not producesignificant local pits or hot spots on the electrodes which in turncauses a current concentration at that spot.

Tests of a laser in accordance with the invention incorporating wirebrush electrodes as generally described hereinabove has indicated thatbroadly the dimensions of each projection should preferably be such thatthe spacing between adjacent projections is approximately equal to aboutone-third the length of the projections in the direction of theelectrode spacing and approximately one-third the width of theelectrodes, i.e., the width of cavity 19.

In one group of tests, wire brush electrodes were provided at the topand bottom of cavity 19. In one case, six strips of No. 30 gauge, 50x50stainless steel wire screen was provided in each electrode and in asecond case only two strips of the same wire screen material wereprovided in each electrode. In both cases, the projections which inthese instances comprised freestanding wires were about one-eighth inchlong. In the case of the electrodes with six strips, development ofsevere hot spots required discontinuance of the test. Upon disassemblyof the channel, it was found that several wires has become weldedtogether. Accordingly, too great a number of projections is likely torender separation of individual projections essentially impossible andthus tends to actually provide many potential points for hot spots. Theresults of tests with the top and bottom electrodes with just two stripsindicated that in this case accurate spacing of the oppositely disposedfreestanding wires is important. Further, the use of projections of asize, magnitude and spacing of that of No. 30 gauge, 50X 50 wire screenis not recommended since freestanding wires of this size are prone tobecome easily twisted and tangled before and after insertion into thedischarge channel.

An embodiment substantially as shown and described herein utilizing No32 gauge, 30x30 stainless steel wire screen with only the base 11water-cooled operated continuously for over 200 hours with a peak outputpower of about 130-140 kilowatts at 100 pulses per second. While intests on this embodiment one hot spot appeared and disappeared in thesame place at infrequent intervals, the appearance of this localized hotspot is believed to have been due to the twisting or contact of two ormore wires at this point. In further tests of a similar electrodearrangement, operation in excess of 100 hours showed no development ofhot spots or other failure mechanism in the laser channel or cavity 19.The laser operated at 100 pulses per second with an output of 100kilowatts peak power and a dissipation of about 200 watts in the cavity.This operation without any adverse effects is, by way of comparison, afactor of 2% times the 40 hours of stable operation that can be expectedwith prior art lasers and electrodes as disclosed, for example, in theaforementioned patent application Ser. No. 536,094. Reference is made tothis patent application for a more complete description and a discussionof the operation of lasers of the type here concerned and described.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims:

1. In a laser device for producing pulses of light at highenergy levelscomprising means defining an elongated cavity with height and widthdimensions having a longitudinal axis; means for supplying to saidcavity a gas having at least two energy levels above the ground energylevel in which thenet excitation rate for the higher of the two upperlevels is greater than the net excitation rate for the lower of the twoenergy levels during electric discharge in said gas; and pulse circuitmeans for connection to a pulsed source of high-voltage electricalpower, said pulse circuit means including first and second opposedelectrode members communicating with said cavity and disposed onopposite sides of said cavity and extending parallel to the longitudinalaxis, said pulse circuit means providing said discharge at a pluralityof different points along the length of said first electrode memberwhereby said discharge is substantially unifon'nly distributed along thelength of said longitudinal axis and across said cavity normal to saidlongitudinal axis within the radiative lifetime of said hi er state,said pulse circuit means having an inductance w ereby a significantfraction of the current in said discharge is delivered within the lengthof the laser pulse duration, the improvement comprising:

a. a plurality of electrically conductive projections extending intosaid cavity from said first electrode member and toward said secondelectrode member, said projections extending along the length of saidcavity and defining a plurality of end surfaces spaced one from anothera distance that is small compared to the length of said cavity, said endsurfaces being small compared to both said width and height dimensionsof said cavity.

2. The improvement as defined in claim 1 wherein said projections extendinto said cavity normal to said longitudinal axis and currentsoriginating from said projections produce a sub stantially uniformcurrent distribution along said longitudinal axis.

3. The improvement as defined in claim 2 wherein the spacing of saidprojections and their cross section is small compared to the spacing ofsaid first and second electrode members whereby said uniform currentdistribution is substantially unaffected by local current concentrationsat said projections.

4. The improvement as defined in claim 1 wherein said projections are ofa wirelike configuration.

5. The improvement as defined in claim 4 wherein said projections definefirst and second rows of consecutive projections extending from saidfirst electrode member toward said second electrode member substantiallythe length of said cavity and disposed, respectively, adjacent oppositesides of said cavity which are proximate to said first electrodes 6. Theimprovement as defined in claim 1 wherein the spacing between adjacentprojections is about one-third the length of said projections and aboutone-third the width dimension of said cavity.

7. The improvement as defined in claim 6 wherein said cavity has endsand said projections intermediate said ends of said cavity are all thesame length and said. projections adjacent said ends of said cavityrespectively decrease in length in the direction of said ends.

8. The improvement as defined in claim 6 wherein said first electrodemember has a, groove coextensive with said cavity, said projectionsdefining said first and second rows comprising respectively freestandingwires forming part of first and second strips of wire screen; andadditionally including:

a. first and second outer electrically conductive spacer means andelectrically conductive middle spacer means, first and second strips ofwire screen being interposed between respectively an outer spacer andthe middle spacer; and means for maintaining said spacers and strips insaid groove.

1. In a laser device for producing pulses of light at highenergy levelscomprising means defining an elongated cavity with height and widthdimensions having a longitudinal axis; means for supplying to saidcavity a gas having at least two energy levels above the ground energylevel in which the net excitation rate for the higher of the two upperlevels is greater than the net excitation rate for the lower of the twoenergy levels during electric discharge in said gas; and pulse circuitmeans for connection to a pulsed source of high-voltage electricalpower, said pulse circuit means including first and second opposedelectrode members communicating with said cavity and disposed onopposite sides of said cavity and extending parallel to the longitudinalaxis, said pulse circuit means providing said discharge at a pluralityof different points along the length of said first electrode memberwhereby said discharge is substantially uniformly distributed along thelength of said longitudinal axis and across said cavity normal to saidlongitudinal axis within the radiative lifetime of said higher state,said pulse circuit means having an inductance whereby a significantfraction of the current in said discharge is delivered within the lengthof the laser pulse duration, the improvement comprising: a. a pluralityof electrically conductive projections extending into said cavity fromsaid first electrode member and toward said second electrode member,said projections extending along the length of said cavity and defininga plurality of end surfaces spaced one from another a distance that issmall compared to the length of said cavity, said end surfaces beingsmall compared to both said width and height dimensions of said cavity.2. The improvement as defined in claim 1 wherein said projections exteNdinto said cavity normal to said longitudinal axis and currentsoriginating from said projections produce a substantially uniformcurrent distribution along said longitudinal axis.
 3. The improvement asdefined in claim 2 wherein the spacing of said projections and theircross section is small compared to the spacing of said first and secondelectrode members whereby said uniform current distribution issubstantially unaffected by local current concentrations at saidprojections.
 4. The improvement as defined in claim 1 wherein saidprojections are of a wirelike configuration.
 5. The improvement asdefined in claim 4 wherein said projections define first and second rowsof consecutive projections extending from said first electrode membertoward said second electrode member substantially the length of saidcavity and disposed, respectively, adjacent opposite sides of saidcavity which are proximate to said first electrode.
 6. The improvementas defined in claim 1 wherein the spacing between adjacent projectionsis about one-third the length of said projections and about one-thirdthe width dimension of said cavity.
 7. The improvement as defined inclaim 6 wherein said cavity has ends and said projections intermediatesaid ends of said cavity are all the same length and said projectionsadjacent said ends of said cavity respectively decrease in length in thedirection of said ends.
 8. The improvement as defined in claim 6 whereinsaid first electrode member has a groove coextensive with said cavity,said projections defining said first and second rows comprisingrespectively freestanding wires forming part of first and second stripsof wire screen; and additionally including: a. first and second outerelectrically conductive spacer means and electrically conductive middlespacer means, first and second strips of wire screen being interposedbetween respectively an outer spacer and the middle spacer; and meansfor maintaining said spacers and strips in said groove.